How to set an image strip (ImageSequence) length via python?
You can use frame_offset_start to change length from left side and frame_offset_left to change length from right side. Building on your code
img_strip = bpy.context.scene.sequence_editor.sequences.new_image(
name = img_name, 
filepath = img_filepath, 
channel = 5, 
frame_start = 1500)

img_strip.frame_offset_start = 10
img_strip.frame_offset_end = -20 # Not sure why but blender needs this value negative. 
#If you put positive value strip will end up in negative side of frames.

#img_strip.frame_start = 20

There is also frame_start setting it does not change the length of the strip but put it at given frame